remove dead links for v1606 and v3+ upgrade guides (no content)

upgrade/v2306-Developer-Upgrade-Guide.md

 <!-- --- title: Developer Upgrade Guide (OpenFOAM-v2306) -->
 
 [![home](/icons/home.svg "wiki home")](/home)
+[![user](/icons/user.svg "user upgrade")](/upgrade/v2306-User-Upgrade-Guide)
 [![upgrade](/icons/chevrons-up.svg "upgrade guide")][upgrade-guide]
 [![code](/icons/code.svg "coding patterns")][code-patterns]
 
@@ -55,7 +56,7 @@ properly detected.  Since the initialised state also has an empty
 states is no longer necessary.
 
 For further convenience, the new `IOobject::resetHeader()` method can
-be used for repeated read operations. It also it enforces resetting of
+be used for repeated read operations. It also enforces resetting of
 headerClassName, scalar/label sizes etc prior to reading. For example,
 ```
 IOobject rio("none", ..., IOobject::LAZY_READ);
@@ -64,7 +65,7 @@ rio.resetHeader("U")
 if (returnReduceOr(rio.typeHeaderOk<volVectorField>(false)))
     ...
 
-io.resetHeader("p")
+rio.resetHeader("p")
 if (returnReduceOr(rio.typeHeaderOk<volScalarField>(false)))
     ...
 ```
@@ -150,7 +151,7 @@ confusing code. For example,
 geofield.clamp_min(lower);
 
 // Old: retains the max of field value and the given value
-// (ie, sets the lower bound)
+// (ie, imposes a min bound)
 geofield.max(lower);
 ```
 In some case, the updated naming helps avoid coding errors.
@@ -177,7 +178,7 @@ improvement.
 ### GeometricField construction
 
 It is now possible to construct a GeometricField from separate
-value/dimensions, which can be easier that copying into a dimensioned
+value/dimensions, which can be easier than copying into a dimensioned
 type (especially when using derived field types). For example,
 
 ```